The process is caused by warmer water temperatures, which cause coral to expel algae that lives in their tissues, turning them white. A coral isn’t dead when it turns white, but it is more endangered. Bleaching can also be triggered by storm runoff and pollution, overexposure to sunlight and exposure to air during low tides.

There are almost 800 species of reef-building corals in all depths of ocean water around the world, with new species being discovered each year. Shallow tropical reefs in the Indian and Pacific Oceans have the greatest number of species. Most reefs are found in tropical and semitropical water within 30 degrees of the Equator.

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Last year, the Great Barrier Reef off the coast of Australia experienced a massive bleaching event that left 67 percent of its shallow-water corals dead. The bleaching comes after severe events in 1998 and 2002, and scientists with the ARC Center of Excellent for Coral Reef Studies fear another one could occur before the reef can recover in the next 10 to 15 years.

The frequency of adverse events impacting coral health has prevented reefs from fully recovering between bleaching. The researchers call for “shaping human-environment interactions through management actions that reduce sensitivity to climate threats.”

The study says that the global climate agreement reached in Paris in 2015 is a good first step at doing this, but “much greater emissions reductions are required to prevent the great majority of coral reefs from experiencing severe bleaching conditions annually within this century.”

“This reinforces the importance of pursuing efforts to limit the temperature increase to 1.5 degrees Celsius,” the study wrote. “This is especially the case given the widespread coral bleaching that has been occurring globally since 2014 with global warming of 0.9 °C. This recent bleaching event and the findings presented here deserve attention in policy discussions at national and international levels.”